1. Field of the Invention
The present invention relates to a supercharged internal combustion engine for an automotive vehicle with a supercharger equipped in an intake system.
2. Description of the Related Art
As a supercharged internal combustion engine so arranged as to increase its power by supercharging intake air into the engine may present the problem that a thermal load within the engine becomes larger, it is proposed in Japanese Patent Laid-open Publication (kokai) No. 3-23,327 that an air-fuel ratio of a mixed fuel to be inhaled into the engine is made leaner than stoichiometric in a supercharging region where the supercharger can exhibit a sufficient degree of its supercharging capability, thereby improving a rate of fuel to be consumed while lowering the temperature within the engine, such as temperatures at valve bridge portions and so on.
It is apparent from FIG. 6, however, that improvements in the rate of consumption of the fuel, which can be achieved by making the air-fuel ratio of the mixed fuel leaner, can be fulfilled at the air-fuel ratio close to an A/F ratio of 15 and that even if the air-fuel ratio is further made leaner, additional improvements in the rate of consumption of the fuel cannot be expected too much. FIG. 6 shows the relationships of the rate of recirculation of the exhaust gases (EGR) and the air-fuel ratio with various factors, such as the temperatures at the valve bridge portion, the temperatures of exhaust gases, etc., in order to gain the same torque by the same engine under the same load. In FIG. 6, the exhaust gases having the temperature of approximately 100.degree. C. were recirculated into the intake system of the engine.
It is found from the foregoing relationships that, even if the air-fuel ratio of the mixed fuel is made leaner than the A/F ratio of 15 in the supercharging region, the improvements to be achieved by such a leaner air-fuel ratio are restricted merely to a decrease in the temperature within the engine.